The propagation of ultra high frequency electromagnetic energy from a transmitter to a mobile receiver, i.e., such as cellular telephones, in an urban environment takes place largely by way of scattering. The transmitter generates radio signals which are reflected by physical terrain features, i.e., usually buildings, and refracted by the inhomogeneities in the atmosphere before reaching a mobile receiver. Accordingly, the signal received by the mobile receiver is the vector sum of many waves that arrive by multiple paths which cause fading of the received electromagnetic energy according to a Rayleigh distribution.
Diversity transmission combats such multipath fading. The basic premise behind diversity transmission is tat the proper combination of a number of different transmission paths all carrying the same information will greatly improve the reliability of communication between the transmitter and the mobile receiver. There are basically three different types of diversity transmissions. Space diversity uses one transmitting antenna and several receiving antennas that are spaced apart so that fading on either antenna is not correlated. Time diversity repeats the same message, after an appropriate time period, between a transmitting antenna and a receiving antenna. Finally, frequency diversity transmits the message between the transmitting and the receiving antenna using several frequencies, sufficiently spaced to have uncorrelated fading, to achieve independent diversity branches.
Space diversity systems have been proposed for use in mobile receivers because of their simplicity of design and ease of manufacture. A system of this type can use one of three different methods for combining the received signals. Maximal-ratio combining method achieves, under ideal operation, the best performance improvement of these methods. However, it requires cophasing circuitry, weighting circuitry, and summing circuitry, which results in a relatively complicated design. The equal-gain combining method, as shown in U.S. Pat. No. 4,386,435 to Ulmer et al., requires cophasing circuitry and summing circuitry, but the weighting circuitry is omitted. For mobile receiver applications, such as cellular telephones, both the maximal-ratio and the equal-gain combining methods are unsuitable because of the technical difficulty in realizing cophasing circuitry having a precise and stable tracking performance in a rapidly changing multiphase fading environment, such as encountered with moving vehicles.
The selection method appears to hold the most promise in mobile receiver applications because of its stable operation in fast multipath fading environments and its simple implementation. In this method, the diversity branch having the highest signal level is selected. However, to date, the cellular telephone industry has not yet applied the selection method at IF to a space diversity system to combat the multipath fading that occurs between the transmitter and a mobile receiver.